Friction, Lubrication, and Wear Technology (1997) Part 8 pdf

(Eq 26) Isotropic elasticity theory has been used to obtain these relationships for dislocation density and correlation parameters. In applying Eq 24, one need not assume that the material is isotropic. Table 1 lists results for both face- and body-centered cubic materials. A common pattern is apparent. For cold-work filings, one finds that (Eq 27) For the most part, the dislocation density is high and in the 10 11 /cm 2 range. Dislocations are highly correlated, with R c close to the average diameter of the subgrain for metallic filings and ZrO 2 debris. Table 1 Results for body-centered cubic materials, face-centered cubic aluminum, and partiall y stabilized cubic zirconia Material <L>, nm n d × 10 -11 /cm 2 R c /<L> < > × 10 4 < > × 10 6 Cr 28.5 3.67 1.79 2.63 1.47 V 20.0 6.56 2.17 3.46 4.05 Mo 26.0 4.60 1.99 3.30 2.72 W 21.5 7.32 1.66 4.56 3.27 Nb 20.0 1.53 1.33 0.84 0.29 Al 32.0 4.41 1.64 2.32 1.60 ZrO 2 8.6 12.70 1.42 4.84 4.90 Aluminum filings are prepared under liquid nitrogen. Source: Ref 18 The results given in Table 1 for body- and face-centered cubic material allow a simple explanation to be given for the structure of cold-worked fragments. Filings show a high degree of correlation (small R c /<L>) and high dislocation densities. This is related to sample conditions, such as the temperature and local conditions at which a high-density dislocation structure is produced. Fragments produced by filing or grinding at room temperature, or even at liquid nitrogen temperature, cannot be "cold." Instead, they are rapidly heated to a high temperature during fracture, and rapidly quenched, because of their high surface-to-volume ratio. Dislocations are mobile for short periods of time over short distances and can cluster. Table 1 includes results from cubic debris particles obtained from a worn PSZ. The substrate contains three phases, as already discussed. It should be noted that the cubic phase is stable at high temperatures, making it reasonable that only the cubic form should be found in the debris. The dislocation density for zirconia is the highest entry, whereas the correlation distance is close to the mean diameter. One can present the same arguments for zirconia debris as for cold-worked metal filings. Prior to breakaway, material at the asperity has a very high dislocation density. A high separation temperature allows some dislocation movement to form subgrain clusters. The question of how much debris one must collect for an ideal line-shape analysis needs to be considered. Typically, for quantitative work, one would like to be able to fill a cavity of at least 15 mm (0.6 in.) in diameter to a depth of 2 mm (0.08 in.). The sample thickness should be several times the penetration depths already discussed. Quantitative diffractometer studies require much more sample than what is required for an x-ray powder camera, but also offer the opportunity for more extensive data analysis. Conclusions and Future Trends Both the wear-modified phase distribution and the debris indicate that at least a fraction of the surface material has been subjected to high temperatures during wear testing. Zirconia debris has an even higher dislocation density than metal filings, indicating severe deformation before breakaway from the surface. High correlation between dislocations is likely to result from a time-restricted, thermally activated process taking place at an elevated temperature immediately after fracture. It was found that large residual compressive strains build up in the ground surface region of the zirconia substrate. A state of dynamic equilibrium is likely to be present between the production of regions of high defect density- residual strain and relaxations produced by annealing. The annealing processes within near-surface bulk substrate material is likely to offer a spectrum of possibilities. Data from a ground PSZ sample revealed the formation of a place gradient extending over a distance of several microns. This represents another form of conditioning that could influence subsequent wear testing. Depths from a few tenths to the micron range are typical x-ray probe distances for many commercial materials. Future research will require a careful selection of samples and radiation. Quantitative diffractometer data will give the greatest amount of information. A part from local surface asperities, the mean surface should be flat. Typically, it also should have surface dimensions ranging from 10 to 30 mm (0.4 to 1.2 in.) to fit into commercial systems. XRD data should be inter-related with XRF or other near-surface analyses revealing chemical changes. Variable penetration depths often allow the condition near a surface to be compared with deeper regions that are relatively unaffected by wear processes. A detailed analysis of diffraction patterns from concentrated industrial wear debris under different conditions may establish trends that predict malfunctions. Similarities or differences between the wear from a given machine and testers, which are used to simulate machine conditions, could also be examined. Thin-film attachments are available for most commercial diffractometers, allowing low glancing angles to be attained. This can greatly reduce the beam penetration. These attachments should be considered for examinations of worn surfaces, as well as for thin layers of debris. Perhaps the most severe restriction in an x-ray analysis using conventional x-ray sources is the limited area of disturbed surface available for examination using routine pin-on-disk testing. One would rather examine square cross sections of at least 15 mm (0.6 in.). Although synchrotron radiation would allow one to examine a small fraction of this size, it is often not readily available. A smaller-sized sample used with conventional sources would force compromises in the data analysis and lessen the opportunity to obtain quantitative results. Any use of x-rays must begin with a consideration of sample size. References 1. G.H. Vineyard, Phys. Rev. B: Condens. Matter, Vol 26, 1982, p 416 2. W.C. McMaster, N. Kerr Del Grande, J.H. Mallett, and J.H. Hubbell, "Compilation of X- ray Cross Sections," Report UCRL- 50174, Sec. I, 1970; Sec. II, Rev. I, 1969; Sec. III, 1969; Sec. IV, Lawrence Radiation Laboratory (Livermore), 1969 3. R.J. Harrison and A. Paskin, Acta Cryst., Vol 17, 1964, p 325 4. B. Hwang and C.R. Houska, J. Appl. Phys., Vol 63, 1988, p 5346 5. C.J. Sparks, Synchrotron Radiation Research, H. Winick and S. Doniach, Ed., Plenum Publishing, 1980 6. B.E. Warren, X-Ray Diffraction, Addison-Wesley, 1969 7. L.H. Schwartz and J.B. Cohen, Diffraction from Materials, Springer-Verlag, 1987 8. B.D. Cullity, Elements of X-Ray Diffraction, Addison-Wesley, 1978 9. B. Hwang, "Near Surface Structure of Ceramic Components," Ph. D. thesis, Virginia Polytechnic Institute and State University, May 1987 10. T.R. Thomas, Rough Surfaces, Longman, London, 1982 11. B. Hwang, C.R. Houska, G.E. Ice, and A. Habenschuss, Adv. Ceram. Mater., Vol 3, 1988, p 189 V 12. R.C. Garvie, R.H.K. Hannink, and N.V. Swain, J. Mater. Sci. Lett., Vol 1, 1982, p 437 13. C.R. Houska, J. Appl. Phys., Vol 41, 1970, p 69 14. C.R. Houska, Treatise on Materials Science, Vol 19A, H. Herman, Ed., Academic Press Inc., 1980 15. B. Hwang, C.R. Houska, G.E. Ice, and A. Habenschuss, J. Appl. Phys., Vol 63, 1988, p 5351 16. C.R. Houska, J. Appl. Phys., Vol 52, 1981, p 748 17. S. Rao and C.R. Houska, Acta Cryst., Vol A42, 1986, p 14 18. S. Rao and C.R. Houska, Matter. Res. Soc. Symp. Proc., Vol 138, 1989, p 93 19. S. Rao and C.R. Houska, Acta Cryst., Vol A44, 1988, p 1021 Basic Tribological Parameters Horst Czichos, BAM (Germany) Introduction TRIBOLOGICAL PARAMETERS are characteristics of mechanical systems with "interacting surfaces in relative motion," including the initiation of motion. The tribological processes of interacting surfaces have a dual character. They are on one hand necessary for the functional performance of "tribosystems" or "tribocomponents" (see the "Glossary of Terms" in this Volume), but are on the other hand inevitably connected with friction and wear. In engineering applications, the functional purpose of trobosystems can be broadly classified into the following categories (Ref 1): • The guidance, transmission, coupling, control, stop, and annihilation of motion, force, mechanical ene rgy, and power (bearings, joints, gears, clutches, cams and tappets, bolts and nuts, fasteners, and brakes) • The transportation and control of flow of matter (pipelines, wheel/rail, tire/road, valves, and seals) • The forming, machining, and tearing of mate rials (drawing, pressing, cutting, shaping, quarrying, and dredging) • The generation and transmission of information (printing heads and magnetic recording interfaces) The diagnosis of friction and wear data of such tribosystems or corresponding laboratory test configurations and test specimens requires special attention because numerous characteristics, parameters, and factors must be taken into consideration. This is due to the fact that friction and wear are not intrinsic materials properties, but must be related to the entire system of interacting components, namely materials pairs and interfacial lubricants. This is obvious from a comparison between the test conditions to obtain strength data or friction and wear data (Fig. 1). Fig. 1 Characteristics and parameters of (a) strength tests and (b) friction and wear tests In strength tests (Fig. 1a), the deformation or fracture resistance of a material specimen in a given environment is determined under the action of a certain stress mode, such as tension, compression, shear, bending, or torsion. The resulting strength data (in terms of force per cross section, or energy) are considered as intrinsic materials properties depending basically on the following groups of parameters: • Materials parameters, such as composition, microstructure, and specimen geometry • Operational parameters, such as stress type, load, deformation velocity, and temperature In a friction or wear test (Fig. 1b), the resistance against motion (friction) or the resistance against surface damage (wear) of a material/material pair (dry system) or a material/lubricant/material combination (lubricated system) in a given environment is determined under the action of a certain type of motion, such as sliding or rolling. The resulting tribometric characteristics in particular the friction or wear data must understood as tribological systems characteristics associated with the following group of parameters: • Structural parameters, which characterize the components (materi als, lubricant, and environment) involved in the friction and wear process and their physical, chemical, and technological properties • Operational parameters, that is, the loading, kinematic, and temperature conditions and their functional duration • Interaction parameters, which characterize, in particular, the action of the operating parameters on the structural components of the tribological system and define its contact and lubrication modes Structural Parameters The analysis of structural parameters must identify first the components involved in a given friction and wear problem. Figure 2 shows typical examples of tribosystems subject to friction and wear together with corresponding simplified test configurations and their elementary structure. This figure illustrates that in any friction and wear situation, four tribocomponents are involved (Ref 2): • Triboelement (1) • Triboelement (2) • Interfacial element (3), for example, lubricant or dust particles • Environmental medium (4), for example, air or corrosive atmosphere Table 1 lists examples of the tribocomponents that make up various tribosystems. Table 1 Structural components of common tribosystems Tribosystem Triboelement (1) Triboelement (2) Interfacial element (3) Environmental medium (4) Type of systems structure Gear box Gear 1 Gear 2 Gear oil Air Closed Wheel/rail Wheel Rail Moisture Air Open Sliding guide Slider Support Grease Air Closed Bearing Bushing Shaft Lubricant Oil mist Closed Dredge Shovel Soil . . . Dust Open Milling system Milling wheel Milling jaw Minerals Air Open Fig. 2 Examples of engineering tribosystems, test configurations, and their elementary structure In analyzing the structure of tribosystems, a distinction can be made between "closed systems," in which all components are continuously involved in the friction and wear process, and "open systems," in which a materials flow in and out of the system occurs. The friction and wear data of tribosystems depend on various properties of their structural components (tribocomponents). Structural parameters of closed tribosystems can be classified in most cases into two groups. Group A consists of triboelements (1) and (2) and involves: • Chemical parameters such as volume composition and surface composition • Physical parameters such as thermal conductivity • Mechanical parameters such as elastic modulus, hardness, and fracture toughness • Geometric parameters such as geometry dimensions, and surface topography • Microstructural parameters such as grain size, dislocation density, and stacking fault energy Group B consists of interfacial (fluid) element (3) and environmental (gaseous) medium (4) and involves: • Chemical parameters such as composition, additive content, acidity, and humidity • Physical parameters such as density, thermal conductivity, and flash and fire point • Mechanical parameters such as viscosity, and viscosity-temperature and viscosity- pressure characteristics For open systems, for example, manufacturing systems such as machining and molding, or quarrying and dredging systems, the structural parameters characterizing the materials flow in and out of the system are often difficult to specify. In addition to the structural elements necessary to fulfill the functional purpose of the tribosystem, detrimental elements such as dirt, dust, and moisture may also be present and must be recognized in the analysis of structural parameters. To assist in the compilation of the various parameters relevant to a given friction and wear problem, a data sheet of basic tribological parameters is described later in this article (see the section entitled "Data Sheet of Basic Tribological Parameters" ). Operational Parameters Operational parameters characterize the functional conditions of a tribosystem. They can be considered (with the exception of friction-induced temperatures) as independent variables that can be varied during tribological testing to obtain friction and wear data experimentally. The basic operational parameters in tribology are: • Type of motion, that is, the kinematics of triboelements (1) and (2), to be classified in terms of sliding, rolling, spin, and impact and their possible superpositions (Fig. 3). The kinematics can be conti nuous, intermittent, reverse, or oscillating • Load (F N ), defined as the total force (including weight) that acts perpendicular to the contact area between triboelement (1) and (2), as shown in Fig. 3 • Velocity (v ), to be specified with respect to the vector components and the absolute values of the individual motions of triboelements (1) ad (2). According to the Table 2 , distinctions must be made among the relative velocity v r (relevant to friction-induced temperature rises), the sum velocity v s (relevant, in lubricated tribosystems, to the formation of an elastohydrodynamic film), and the slide-to- roll ratios • Temperature (T) of the structural components at stated location and time, that is, the initial (steady- state) temperature and the friction- induced temperature rise (average temperature rise and flash temperatures) to be estimated on the basis of friction heating calculations (see the following article in this Section on "Design of Friction and Wear Experiments") • Time dependence of the set of operational parameters (F N , v, T, for example, load cycles and heating or cooling intervals • Duration (t) of operation, performance, or test In addition to these functional operational parameters, disturbances such as external vibrations or radiation might need to be taken into consideration as well. Table 2 Type of motion and velocities of the components of a tribosystem for sliding and sliding and rolling Fig. 3 Kinematics of tribosystems Interaction Parameters Interaction parameters characterize the action of the operational parameters on the structural components of tribosystems. These parameters define in particular the contact mode and the lubrication mode of a tribosystem with a given material/material or material/lubricant/material structure. The contact mode of two touching solid bodies is characterized microscopically by materials interactions, which are described by contact stresses and stress distributions. The materials and stress interactions cause a resistance against motion (friction) and may lead to surface damage (wear). Therefore, the materials and stress interactions in tribosystems are also called friction and wear mechanisms, or generally tribological processes, and specified in terms such as adhesion, abrasion, tribochemical reactions, surface fatigue, and so forth. Interface Forces and Energies. Theoretically, the microscopic interaction forces between contacting solids include, at least in principle, all those types of atomic and molecular interaction that contribute to the cohesion of solids, such as metallic, covalent, and ionic, that is, primary chemical bonds (short-range forces), as well as secondary van der Waals bonds (long-range forces) (Ref 3, 4). These surface forces depend in a complicated manner on the physicochemical nature of the materials and the structure and composition of the outermost surface layers and contaminants. It should also be noted that the chemical composition, the electronic nature, and the microstructure of surfaces may be quite different from that of the subsurface (volume) of a material. Experimentally, the only macroscopic way to characterize adhesive interactions between two solid bodies contacting under a normal load, F N , is to destroy the bonding and to measure in the opposite direction to F N the force, F A , necessary for the separation of the surfaces. The ratio a = F A /F N is termed the coefficient of adhesion. On the microscopic level, it is possible to determine with an atomic force microscope (noise level 2 × 10 -11 N) the interface forces (including friction forces) between single atoms of contacting surface tips (Ref 5). In energetic terms, the formation of a solid/solid contact results in a net release of surface energy resulting from the replacement of two surfaces by one solid/solid interface of lower surface energy. The change in surface energy per unit area of contact, , can be written as: [...]... the Science and Technology of Friction, Lubrication and Wear, Elsevier, Amsterdam, 19 78, p 351-353 2 "Wear: Terms, Systems Analysis of Wear Processes, Classification of the Field of Wear, " DIN Standard 50 320, Beuth-Verlag, Berlin, Dec 1979 3 N.P Suh, Tribophysics, Prentice-Hall, 1 986 , p 26-45 4 D.H Buckley, Surface Effects in Adhesion, Friction, Wear, and Lubrication, Elsevier, Amsterdam, 1 981 , p 245-312... common contact area Wear volumes are connected via density or specific gravity with wear masses or wear weights In addition to these quantities, a wear- time ratio may be defined as wear velocity (Ref 15) Other common wear parameters are the wear rate, which is the wear volume per unit of sliding distance, and the wear coefficient, k, which is defined as: (Eq 12) where W is the wear volume (in mm3),... Vol 92, 188 1, p 156 (in German) 9 R.D Mindlin, Compliance of Elastic Bodies in Contact, J Appl Mech (Trans ASME), Vol 71, 1949, p 259 10 E Broszeit, T Preussler, M Wagner, and O Zwirlein, Stress Hypotheses and Material Stresses in Hertzian Contact, Z Werkstofftech., Vol 17, 1 986 , p 2 38- 246 11 H Czichos, Tribology A Systems Approach to the Science and Technology of Friction, Lubrication and Wear, Elsevier,... 1 and ) can be taken from the Table 2 Chart of x2 values (n number of individual observations, 1 n 3 5 7 10 15 20 25 30 40 60 = 0.10 5.991 9.4 48 12.592 16.919 23. 685 30.144 36.415 42.557 54.572 77.931 = 0.05 0.103 0.711 1.635 3.325 6.571 10.117 13 .84 8 17.7 08 25.695 42.339 7.3 78 11.143 14.449 19.023 26.119 32 .85 2 39.364 45.722 58. 120 82 .117 2 distribution (Table 2) confidence level) = 0.01 0.051 0. 484 ... 82 .117 2 distribution (Table 2) confidence level) = 0.01 0.051 0. 484 1.237 2.700 5.629 8. 907 12.401 16.047 23.654 39.662 10.597 14 .86 0 18. 5 48 23. 589 31.319 38. 582 45.559 52.336 65.476 90.715 0.010 0.207 0.676 1.735 4.075 6 .84 4 9 .88 6 13.121 19.996 34.770 Design of Experiments Mathematical models, computer simulations, and statistical techniques are of fundamental importance for the appropriate design of... tribological processes as well as changes in the composition and microstructure of the triboelements and the resulting shape and composition of wear surfaces and wear particles must be characterized (see the Sections of this Handbook that deal with laboratory characterization techniques) It is important to recognize, however, that friction and wear parameters are system-dependent characteristics that... studies of friction and wear processes The conditions of these tests are often selected to study specific tribological phenomena rather than to simulate real triboengineering behavior Laboratory Friction and Wear Tests and Simulative Tribotesting The basic characteristics and relevant parameters of laboratory and simulative tests are shown in Fig 2 The design of laboratory friction and wear tests should... 245-312 5 G.M McClelland and S.R Cohen, "Tribology at the Atomic Scale," Research Report RJ 7444, IBM, 26 Apr 1990 6 D Tabor, A Simplified Account of Surface Topography and the Contact between Solids, Wear, Vol 32, 1975, p 269 7 A Majumdar and B Bushan, Role of Fractal Geometry in Roughness Characterization and Contact Mechanics of Surfaces, J Tribol (Trans ASME), Paper 89 -Trib-20 8 H Hertz, On the Contact... to characterize measured friction and wear data are the mean and the standard deviation (Ref 2) If y1, y2, ., yn, are denoted the n individual observations of a random sample from a tribotest, the mean ( ) and standard deviation (s) are defined as follows: (Eq 1) (Eq 2) While the mean characterizes the location of the observed values on a corresponding scale, the standard deviation gives a measure... Three main lubrication regimes can be distinguished by Stribeck curves (Ref 12) Figure 8 shows how lubrication regimes and variations of friction and wear coefficients are functions of the parameter combination · v · , or the film thickness-to-roughness ratio Fig 8 Lubrication regimes and variations of friction and wear coefficients as functions of the ratio of film thickness to roughness Regime I: . Acta Cryst., Vol A42, 1 986 , p 14 18. S. Rao and C.R. Houska, Matter. Res. Soc. Symp. Proc., Vol 1 38, 1 989 , p 93 19. S. Rao and C.R. Houska, Acta Cryst., Vol A44, 1 988 , p 1021 Basic Tribological. composition and microstructure of the triboelements and the resulting shape and composition of wear surfaces and wear particles must be characterized (see the Sections of this Handbook that. 1 982 11. B. Hwang, C.R. Houska, G.E. Ice, and A. Habenschuss, Adv. Ceram. Mater., Vol 3, 1 988 , p 189 V 12. R.C. Garvie, R.H.K. Hannink, and N.V. Swain, J. Mater. Sci. Lett., Vol 1, 1 982 ,